The present application relates to the field of radiation imaging systems. It finds particular application to data acquisition systems of radiation imaging systems that use photon counting detector arrays to measure a number and/or energy of radiation photons impinging thereon.
Today, radiation imaging systems such as computed tomography (CT) systems, single-photon emission computed tomography (SPECT) systems, projection systems, and/or line-scan systems, for example, are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation comprising photons (e.g., x-rays, gamma rays, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by interior aspects of the object, or rather an amount of radiation photons that is able to pass through the object. Generally, highly dense aspects of the object absorb and/or attenuate more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or metal, for example, may be apparent when surrounded by less dense aspects, such as muscle or clothing.
Radiation imaging systems typically comprise a detector array having one or more detector cells. Respective detector cells are configured to indirectly or directly convert radiation photons impingent thereon into electrical charge which is used to generate an electrical signal. The detector cells are typically “energy integrating” or “photon counting” type detector cells (e.g., the imaging system operates in energy integrating mode or photon counting mode).
Energy integrating detector cells are configured to convert radiation energy into electrical charge. The charge generated over a period of time (e.g., at times referred to as a measurement interval) is integrated to generate a signal that is proportional to an incoming radiation photon flux rate at a detector cell. While energy integrating detector cells are widely used, there are several drawbacks to this type of cell. For example, energy integrating detectors cells are generally not able to provide feedback as to the number and/or energy of radiation photons detected. As another drawback, there is a lower limit of detection defined by noise such that a detector cell with little to no incident radiation may produce some signal due to thermal and/or analog read noise (e.g., produced by a radiation conversion element and/or electronics arrangement of the detector cell). It may be appreciated that as a result of this lower limit, the dose of radiation that is applied to an object under examination is generally greater than the dose of radiation that may be applied to the object if the detector cells are of a photon counting type.
Photon counting type detector cells are configured to output a signal (e.g., a pulse) for respective detected radiation photons (e.g., where the detection of a radiation photon may be referred to as a detection event). In some embodiments, the signal (e.g., amplitude of the pulse) is indicative of a radiation energy of the detected radiation photon. A controller is configured to determine the location and energy of respective detected radiation photons based upon the pulse, accumulate the detection events occurring during a measurement interval, digitize the information, and/or process the digital information to form an image, for example. It may be appreciated that there are numerous advantages to photon counting type detector cells over energy integrating detector cells. For example, the counting of radiation photons is essentially noise free (apart from inherent photon shot noise). Therefore, a lower dose of radiation may be applied to the object under examination. Moreover, photon counting cells generally allow for energy or wavelength discrimination.
While photon counting type detector cells have numerous advantages over energy integrating detector cells, photon counting type detector cells have not been widely applied in some imaging modalities due to, among other things, saturation issues (e.g., pulse pile-up) at high radiation flux rates. For example, CT systems generally detect as many as 109 radiation photons per millimeter squared of a detector per second and can detect radiation photons at even higher flux rates. At such high flux rates, the photon counting type detector cells may be unable to return to a normal state between the detection of a first radiation photon and a second radiation photon, which may cause two detection events to be counted as a single, higher energy event.